1. Field of the Invention
The present invention is concerned with supplemental medicinal gas delivering devices designed to supply to a patient effective amounts of medicinal gases such as oxygen. Devices of this character are commonly used in the treatment of various lung ailments, for example, emphysema. More particularly, the invention is concerned with a greatly improved gas therapy apparatus which is designed to save substantial quantities of medicinal gas, as compared with the conventional and time-honored approach of simply supplying a continuous stream of such gas to the patient. This savings is accomplished by controlling the length as well as the timing of the periods during which oxygen is supplied to the patient in a unique manner in response to the timing of the patient's own breathing efforts. Moreover, the invention comprehends a unique flow control valve designed for use in such gas therapy applications, together with electronic circuitry to provide precise, reliable control under normal and extraordinary conditions encountered in the treatment of patients.
2. Description of the Prior Art
Many patients suffering from diseases of the respiratory system are treated through the use of supplemental oxygen or other gas. Very commonly, and particularly in the case of supplemental oxygen, the administration technique has involved nothing more than applying a continuous stream of oxygen to the patient through a nose cannula or similar device. Thus, a physician may prescribe a given flow rate of oxygen for a patient, and a simple control valve is set to deliver the prescribed flow rate.
While this approach does provide the patient with supplemental oxygen, it is extremely wasteful from the standpoint of oxygen usage. That is to say, physiological studies have demonstrated that much of the oxygen delivered to a patient during a breathing cycle is wasted; it may simply be directly exhaled or never reach the lungs. In fact, prior studies have established that the physiological equivalent of continuous oxygen delivery can be achieved by administering relatively short, high flow rate pulse volumes of oxygen at the beginning of the inspiration cycle, and that if properly done such a therapy is just as effective as continuous administration of relatively lower flow rates of gas.
Apart from the gas waste inherent in continuous systems, it is also generally necessary to humidify a gas which is being continuously delivered to a patient. This may be accomplished by bubbling the gas through a humidifier prior to delivery to the cannula. While such humidifiers are well known, they present a problem inasmuch as the moist environment of the humidifier can be a significant vector for the transmission of infection. This problem can be especially acute in the case of weak or non-ambulatory patients, as will be readily appreciated.
U.S. Pat. No. 4,457,303 to Durkan describes a respirator apparatus designed for intermittent demand oxygen flow and apneic event detection. The structure described in this patent provides a selectively settable device which, at an appropriate time during a patient's breathing cycle, will deliver a predetermined quantity of gas in the form of a fixed time duration pulse. A prime deficiency of this approach, however, is the fact that the device cannot automatically adjust the pulse volume delivered over time to accommodate different patient breathing rates and conditions. Thus, the Durkan apparatus is preset to deliver a constant, predetermined pulse volume, and does so notwithstanding variations in demand on the part of the patient or other external factors Thus, if the patient begins to breathe rapidly, the effective result may be a near-continuous flow of gas, whereas if the patient breathes more slowly only relatively small, widely spaced doses are delivered. In short, the Durkan device provides no "on-the-go" operational flexibility, but rather supplies a constant dose of oxygen under all conditions once set.